1. Field of Invention
The present invention relates to a device and method for use in information transmission via an optical fiber and an optical communication system incorporating the device. More specifically, the present invention relates to a device and method for transmitting multilevel and hybrid encoded information via an optical fiber and an optical communication system incorporating the device.
2. Discussion of Related Art
Demand for optical communication systems is growing with the growing demand for faster and more reliable broadband networks. Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical communication systems. Such optical communication systems include, but are not limited to, telecommunication systems, cable television systems (CATV), and local area networks (LANs). An introduction to the field of Optical Communications can be found in “Optical Communication Systems” by Gowar, ed. Prentice Hall, N.Y., 1993.
WDM optical communication systems carry multiple optical signal channels, each channel being assigned a different wavelength. Optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, and transmitted over a single waveguide such as an optical fiber. The optical signal is subsequently demultiplexed such that each individual channel is routed to a designated receiver.
Information to be transmitted via an optical fiber may be encoded using different coding schemes, including on-off-keyed coding schemes and multilevel coding schemes. Both encoding schemes provide different signal levels to represent different symbols in the information stream. In the former case, the underlying information stream has only two symbols (“on” and “off”). In the latter case, more than two different signal levels are used to represent different symbols. Information encoded using different schemes may be transmitted as a single hybrid stream in the same wavelength channel. The optical signal to noise ratio required to transmit multilevel or hybrid signals is much higher than that required to transmit conventional on-off-keyed signals.
To transmit a large bandwidth of multilevel signals through an optical transmission line, multiple wavelengths can be used. Optical amplifiers may be needed to increase the propagation distance to more than what would be allowed by the natural attenuation of the fiber or of the components added to the communication channels to control the signal. The optical amplifiers increase the power of signals carried in different wavelength channels when such signals pass through the optical fiber.
When an optical amplifier amplifies an optical signal comprising more than one wavelength, variations in gain amplitude as a function of wavelength may lead to degradation of the signal performance in the channels with less power. This problem becomes more severe when multilevel or hybrid signals are present due to the fact that the signal to noise ratio requirement for multilevel signals is much higher than that for conventional on-off-keyed signals.